Vehicle hydraulic brake system operating with power assistance

ABSTRACT

The invention relates to a vehicle hydraulic brake system with a twin-circuit emergency brake pressure source which can be put into operation by a pedal and with wheel brakes which can be supplied from this emergency brake pressure source through emergency brake main conduits and with a power-assisted brake device which has a power source, and, between the latter and the at least one wheel brake, at least one valve arrangement generates brake pressures and also reduces brake pressure using a pressure medium from the power source. At least one valve arrangement is installed between the emergency brake pressure source and the wheel brakes and, in emergency brake operation, this valve arrangement connects the emergency brake pressure source to the wheel brakes and, in power-assisted brake operation, separates the emergency brake pressure source from the wheel brakes. This valve arrangement is designed in such a way that in power-assisted brake operation, the valve arrangement permits pressure medium which has to be drained from wheel brakes to escape to a reservoir of the emergency brake pressure source through the emergency brake main conduits and a return conduit. This valve arrangement can be controlled by a small control force and is therefore of low cost. The vehicle brake system is suitable for motor vehicles.

The invention is based on a vehicle brake system as set forthhereinafter.

A hydraulic power-assisted brake system integrated in a wheel brake isknown from the publication EP 0 025 714 A1. This has a power source witha reservoir, with a pump and with a pressure accumulator and anelectrically controllable brake pressure setting valve, with anelectromagnet and a brake pedal with an electrical sensor, and thesystem has an anti-lock control unit for controlling the brake pressuresetting valve. An electrical converter which acts on the brake pressuresetting valve can be adjusted by actuating the brake pedal. If thedanger of wheel lock occurs when a brake pressure is set by actuation ofthe brake pedal, by means of the brake pressure setting valve, theanti-lock control unit drives the brake pressure setting valve againstwhat is specified by the converter in order to reduce the brake pressureuntil such time as the danger of wheel lock is obviated. In the case ofa failure of an electrical current supply to the pump and/or to theelectrical converter and/or even to the anti-lock control unit, therelevant wheel brake fails. If the remaining wheel brakes of the samevehicle are connected to the same electricity supply, a failure of theelectrical current supply causes a complete failure of the vehiclehydraulic brake system.

Vehicle hydraulic brake systems are known from the publications EP 0 265623 A2, DE 40 29 793 A1, DE 31 31 856 A1, U.S. Pat. No. 4,462,642 andU.S. Pat. No. 4,902,075 which normally operate as electricallycontrollable power-assisted brake systems but which can be operated bymuscle force in the case of a failure of power assistance by using, ineach case, a twin-circuit emergency brake pressure generator which is,for example, configured as a normal main brake cylinder and which can beactuated by means of a brake pedal, with the support, for example, of avacuum-operated servo brake, as is revealed in DE 40 29 793 A1. Inaccordance with EP 0 265 623 A2, electrically operating sensors intendedfor the electrical control of brake pressures are configured, forexample, as a pedal force sensor (which is mechanically coupled to thebrake pedal) or as a pressure sensor (which is connected to theemergency brake pressure generator). So-called displacement simulatorsare connected to the emergency brake pressure generators so that thedriver experiences brake pedal displacements of comfortable lengthduring power-assisted brake operation. In accordance with EP 0 265 623A2 or DE 31 31 856 A1, for example, these displacement simulators can beswitched on and off by valves and include cylinders, simulator pistonsand simulator springs.

A vehicle hydraulic brake system with a main brake cylinder, which isknown from the publication DE 41 02 496 A1, is further developed inorder to generate wheel brake cylinder pressures with the aid of atleast one pump and at least one change-over valve arrangement which canbe controlled electrically. The electrical control takes place on thebasis of signals which are proportional to the forces exerted on thebrake pedal. Apart from the force or pressure sensors quoted for thispurpose, DE 41 02 946 A1 mentions resistive and inductive displacementpick-ups and the measurement of a pedal actuation displacement. In avehicle brake system operating with power assistance and using magneticvalves, EP 0 389 993 A1 reveals setting the brake pressure as aspecified ratio of the brake pedal stroke. A brake pedal stroke detectorprovided for this purpose is represented as a potentiometer with aresistance track curved around the pivoting axis of the brake pedal.

Electrically controllable valves for the setting and/or relay of brakepressures and for shutting off the emergency brake pressure generators,and arranged between such power sources, the emergency brake pressuregenerators and the wheel brakes, can be configured in various ways as2/2-way valves, 3/2-way valves, 3/3-way valves and 4/4-way valves, asrevealed in EP 0 265 623 A2 and DE 31 24 755 A1. In contrast, a specialfeature of the vehicle brake system of DE 40 29 793 A1 consists in thefact that such 2/2-way valves, which are used for increasing andreducing brake pressures, are configured as throttling directionalvalves and can therefore be driven into an infinite number ofintermediate positions between two outer end positions (infinitepositioning). This has the advantage that different pressure changerates can be generated without hydraulic jolts in the wheel brake bymeans of variable throttling. In addition to the steplessly adjustable2/2-way valves, 3/3-way valves are mentioned. The latter are configuredas seat valves because of the leak-tightness necessary in the shut-offposition. Pressure sensors for measuring the wheel brake pressures arepresent so that, by comparing the indications of these pressure sensorswith the indication of a pressure sensor connected to the main brakepressure generator and taking account, for example, of an amplificationfactor which can be increased, brake pressures can be regulated byswitching on a control unit which acts on the valves.

Apart from providing servo operation, the control unit can beadditionally arranged to carry out anti-lock protection and drive-sliplimitation functions and to carry out vehicle dynamics adaptation bymeans of the vehicle brake system.

For vehicle brake systems, EP 0 265 623 A2 provides the functions ofservo brake, anti-lock protection and drive-slip control functions and,in addition, braking force distribution (control power distribution),stop braking (stop retention), distance braking (vehicle-to-vehicledistance control) and braking before an obstacle (obstacle avoidance) aswell as automatic braking for various purposes.

For a brake system which operates with power assistance and which alsohas a brake cylinder actuated by brake pedal, the Japanese PublicationNo. 49,474/79 of 18 Apr. 1979 in accordance with the Japanese PatentApplication No. 115,796/77 of 27 Sep. 1977, reveals electricallycontrollable 4/3-way proportional/directional valves for setting brakepressures.

Many of the known vehicle brake systems have, inter alia, the featurethat pressure medium return conduits are connected to electricallycontrollable wheel brake pressure reduction valves. These returnconduits lead to reservoirs with fluid levels above the emergency brakepressure generators and are therefore designed as so-called openablebrake circuits. The technical complication involved in the pressuremedium return conduits and in laying them in the vehicles so that theyextend above the fluid levels of the reservoirs is disadvantageouslylarge, particularly where the elements used for the power-assisted brakeoperation, such as directional valves, can only be accommodated remotefrom the emergency brake pressure generators. Advantages of theinvention

The vehicle brake system according to the invention has the advantagethat quantities of pressure medium which were previously supplied fromthe power source and which have to be led away from the wheel brakesduring power-assisted operation or at the end of power-assisted brakeoperation, flow through the respective emergency brake conduit as far asthe valve arrangement for selecting the brake pressure source and, fromthere, finally reach the reservoir through the special return conduit.It may be seen that essentially only flow resistances have to beovercome during such drainage of fluid to the reservoir and it maytherefore be recognized that during power-assisted brake operation, theemergency brake conduits can never attain the pressure which is storedin the power source. In consequence, the valve arrangements installed inthe emergency brake conduits for selecting the brake pressure source canbe provided with increased flow cross-sections and stronger valvesprings and stronger electromagnets are not required.

Advantageous further developments and improvements to the vehicle brakesystem are possible by means of the measures listed herein. Theinventive features provide a low-cost construction of the valvearrangement which is used for generating and changing brake pressuresduring power-assisted brake operation. This valve arrangement consistsof so-called two-position valves constructed in a simple manner. Theinventive features further provide the advantage that relatively weakvalve springs and, accordingly, relatively weak electromagnets can beused because the by-passes and the non-return valves permit quantitiesof pressure medium to drain to the emergency brake pressure source whenthe pedal is released even though the respectively associated andnormally open 2/2-way valve may no longer take up its widest openposition because of flow through it. The inventive features provide afurther configuration possibility for those valve arrangements which areused for selecting the brake pressure source. The inventive featuresalso provide a low-cost embodiment example which uses 2/2-way valves. Afurther advantage resides in that it is possible to control rates ofpressure increase and rates of pressure decrease for the wheel brakes ina practically infinitely variable manner and, in consequence, withoutjolts. A further embodiment sets forth a valve arrangement by means ofwhich brake pressures can be generated or set. A still further advantageresides in that brake pressure rates which increase and brake pressurerates which decrease can, for example, be changed in an infinitelyvariable manner and, in consequence, practically without jolts. Theinvention provides the advantage, known per se, of good sealing of thevalve arrangements and simultaneously ensure that compensation isprovided for flow cross-sectional contractions, with disadvantageousflow hindrance, possibly caused by flow through the unit.

In accordance with a principle of action known per se, an intact brakecircuit is prevented from being emptied by another brake circuit whichhas developed a leak. A further development with provides the advantagethat two normally closed 2/2-way valves are arranged in series betweenthe power source and each of the wheel brakes to be supplied from it.This provides the advantage of improved security against unintentionalfilling of the wheel brakes by means of fluid from the power source. Onthe other hand, the use of a spool valve, for example, within the valvearrangement used for brake pressure generation or brake pressure change,this spool valve being very suitable for further development as aproportional valve.

Another is that rates of increase of pressure in the wheel brakes can bechanged in steps in a relatively low-cost manner. In this way, it ispossible to economize in the costs involved in the further developmentof directional valves to provide proportional directional valves.

Further features provide the advantage that elastic flexibility of thebrake pedal is present during power-assisted brake operation; thisfacilitates sensitive setting of brake pressures. A low-cost design isset forth for generating required brake pressure values based on thefact that a desired elastic flexibility of the pedal is effected. It maybe seen that a power-assisted brake system can be further developed foradditional modes of operation with little additional technicalcomplication.

BRIEF DESCRIPTION OF THE DRAWINGS

The vehicle hydraulic brake system according to the invention is shownin the drawing and is explained in more detail in the followingdescription. FIG. 1 shows a circuit diagram for the vehicle brake systemaccording to the invention, FIGS. 2 and 3 show connections for thedirectional valves configured as seat valves and FIGS. 4, 5, 6 and 7shows configurations of valve arrangements which can be alternativelyinstalled in the vehicle brake system of FIG. 1.

DESCRIPTION OF THE EMBODIMENT EXAMPLES

The vehicle hydraulic brake system 2 shown in the circuit diagram ofFIG. 1 has a pedal 3, a pedal rod 4, a so-called vacuum-operated servobrake 5, for example, an emergency brake pressure source 6 which isconfigured, for example, in the form of a commercially availabletwin-circuit brake cylinder with a reservoir 7, four wheel brakes 8, 9,10, 11, for example, which can be arbitrarily distributed among thefront and rear wheels of a vehicle, emergency brake main conduits 12, 13laid in twin circuits, emergency brake conduits 14 to 17 connected tothe emergency brake main conduits 12, 13, wheel brake conduits 18, 19,20, 21 and, as the power-assisted brake device 22, a power source 23with a driving motor 24, a pump 25, a non-return valve 26 downstream ofthe pump 25, a power reservoir 27 in the form of an accumulatordownstream of the non-return valve, a pressure monitor 28 communicatingthe power reservoir 27 and with a safety valve 26a connected to thepower reservoir 27.

One valve arrangement 29, 30 for the selection of a brake pressuresource is provided for each brake circuit. For the four wheel brakes 8to 11, there are, for example, four wheel brake pressure sensors 31, 32,33, 34, a required brake pressure value generator 35 controlled by meansof the pedal 3, a control unit 36 connected to the required brakepressure value generator 35 and the wheel brake pressure sensors 31 to34, four valve arrangements 37, 38, 39, 40 which are electricallycontrollable by the control unit 36, a branch 41 supplied from the powersource 23 and, starting from the branch 41, two power conduits 42, 43,normally closed 2/2-way valves 44 and 45 with by-passes 46 and 47respectively installed in the power conduits 42 and 43 and throttles 48and 49 respectively located in the by-passes 46 and 47, and non-returnvalves 50 and 51 which are preferably located downstream of the 2/2-wayvalves 44 and 45 and their by-passes 46 and 47 and which can be openedby means of pressure from the power source 23, a displacement simulator52 and a displacement simulator connecting valve 53 connected with themain conduit 12.

As shown in FIG. 1, the valve arrangements 29, 30 contain normally open2/2-way valves 54 which are installed in the emergency brake mainconduits 12, 13 and which are, for example, electrically controllable bymeans of electromagnets 55 and they contain further 2/2-way valves 56which are introduced between the emergency brake main conduits 12 and 13and the reservoir 7, which are normally closed and which can likewise becontrolled by electromagnets 58. There is a return conduit 59 betweenthese 2/2-way valves 56 and the reservoir 7.

A valve chamber 60, for the 2/2-way valve 54, with a connection A and aconnection B is represented in a simplified manner in FIG. 2. Theconnection A is associated with a valve seat 61 and a connection B isassociated with a valve closing element 62, for example a ball, whichcan be pressed onto the valve seat 61, and a valve space 63 whichaccommodates the valve closing element 62. In a manner which supportsthe-invention, each connection A is connected to the emergency brakepressure source 6 by the emergency brake main conduit 12 or 13. Viewedfrom the direction of the emergency brake pressure source 6, therespective connection B is correspondingly located behind the respective2/2-way valve 54.

The 2/2-way valve 56 is diagrammatically represented in an analogousmanner in FIG. 3. It again has a valve chamber 60, a valve seat 61, avalve closing element 62 and a valve space 63. A difference from thenormally open 2/2-way valve 54 of FIG. 2 is that the 2/2-way valve 56 ofFIG. 3 is normally closed. This can be recognized in FIG. 3 because thevalve closing element 62 is seated on the valve seat 61. In a similarmanner, a connection associated with the valve seat 61 is againdesignated by A and a connection opening into the valve space 63 isagain designated by B. In a manner which supports the invention, theconnections A of the two valves 56 are connected to the reservoir 7. Incontrast, the connections B of these 2/2-way valves 56 are connected tothe respective emergency brake main conduit 12 or 13. This allocation ofthe connections provides the advantage that, for example, an emergencybrake pressure in the emergency brake main conduits 12 and 13 loads the2/2-way valves 56 in the closing direction and, by this means, avoidsthe danger of a disadvantageous drainage of pressure medium from theemergency brake pressure source 6 in the direction of the reservoir 7.In the case of the 2/2-way valve 54 installed in the emergency brakemain conduits 12 and 13, the arrangement has the advantage that arapidly generated pressure in the emergency brake pressure source 6 actsin the opening direction on the 2/2-way valves 54 so that a reliablesupply is possible to the wheel brakes 8 to 11.

By-passes 64 with built-in non-return valves 65 are laid around thenormally open 2/2-way valves 54. The non-return valves 65 are theninstalled in such a way that they can be opened in the direction towardsthe emergency brake pressure source 6. Installing the non-return valves65 and the by-passes 64 in this way is advantageous to the extent that,in the case of an abrupt reduction of the emergency brake pressure bysudden release of the pedal 3, fluid emerging from the wheel brakes 8 to11 flows back rapidly to the emergency brake pressure source 6 andhydrodynamic forces then occur which may possibly act in the closingdirection on the valve closing elements 62 with the danger, which cannotbe completely excluded, that these elements close if, for some reason,the opening springs have a weak configuration or if the opening springsare broken. In such a case, a reduction of emergency brake pressure ispossible by means of the respective non-return valve 65.

In FIG. 1, the 2/2-way valves 54 have electromagnets 55 to actuate themand the control unit 36 acts on these electromagnets 55. Forcompleteness, it should be mentioned that hydraulic control inaccordance with the prior art is also possible with the aid of pressurefrom the power source 23, instead of control of the electromagnets 55.In accordance with the possibilities quoted of selecting hydraulic orelectromagnetic control, the displacement simulator connecting valve 53can be opened by its own electromagnet 66 by means of the control unit36 for the power-assisted brake operation, as is apparent from thepublication DE 31 31 856 A1, in particular. Alternatively, however, ashas already been mentioned with respect to the 2/2-way valve 54, thedisplacement simulator connecting valve 53 can be likewise controlledhydraulically. The possibility of selecting between electrical controland hydraulic control also applies, of course, to the 2/2-way valves 56,which can have the electromagnets 58 already mentioned and which areopened towards the return conduit 59 and to the reservoir 7 forpower-assisted brake operation.

Each of the valve arrangements 37, 38, 39 and 40 consists, in theembodiment example of FIG. 1, of a normally open 2/2-way valve 70between the valve arrangement 29 or 30 and the respective brakes of thewheel brakes 8 to 11. The 2/2-way valve 70 has an electromagnet 71 forits operation by means of the control unit 36 in power-assisted brakeoperation. In addition, the valve arrangements 37 to 40 each have asecond 2/2-way valve 72 which is normally closed and which can be driveninto the open position by the control unit 36 by means of anelectromagnet 73. As in the case of the 2/2-way valves 54 and 56, theletters A provide the valve connection at the valve-seat end and theletters B the connections at the valve-space end. Because theconnections A have a connection at the power source end, their openingprocedure is supported by the pressure from the power source 23 in thedirection towards the respective wheel brake 8, 9, 10 or 11. Because ofthe connection of the valve connections A of the normally open 2/2-wayvalves 70 via the wheel brake conduits 18 and 19, 20 and 21 to therespective wheel brake 8, 9, 10 or 11, pressure drops act from the wheelbrakes 8, 9, 10, 11 mentioned in the direction towards the emergencybrake conduits 14 and 15, 16, 17 and also, in consequence, towards theemergency brake main conduits 12 and 13 so as to support the openpositions of these 2/2-way valves 70. Because, in consequence, apressure drop from pressures present, during emergency brake operation,in the emergency brake main brake conduits 12 and 13 towards the wheelbrakes 8 and 9, 10, 11 could act in the closing direction in the 2/2-wayvalves 70 when the pedal 3 is actuated, by-passes 75 containingnonreturn valves 74 are laid around these 2/2-way valves 70.

The overall result is that the valve arrangements 37 to 40 have, inpractice, three undesignated connections towards the outside which areconnected to the emergency brake conduits 14 to 17, the wheel brakeconduits 18 to 21 and the two non-return valves 50 and 51.

The respective valve arrangements 37 to 40 are also connected to thewheel brake pressure sensors 31 to 34 by the wheel brake conduits 18 to21.

As already indicated, the supply to the valve arrangements 37 to 40takes place through the non-return valves 50 and 51 which, as alreadymentioned, are supplied with power through the power conduits 42 and 43and the branch 41. The non-return valves 50 and 51 have the effect thatonce pressure medium has been introduced into the wheel brakes 8 to 11,it cannot pass back into the power source 23.

At the same time, these non-return valves 50 and 51 also preventpressure medium from draining unintentionally from the wheel brakes 8and 9, for example, into the wheel brakes 10 and 11 or vice versa. Thisalso achieves the effect that should the wheel brake conduit 20 of thewheel brake 10 develop a leak, for example, this does not empty thewheel brakes 8 and 9.

In the usual manner, the throttles 48 and 49 have the effect thatpressure medium flowing from the power reservoir 27 to the non-returnvalves 50 and 51 can only increase up to a desired magnitude. On theother hand, the 2/2-way valves 44 and 45 which are respectivelyconnected in parallel with the throttles 48 and 49 can, when open, formby-passes around the throttles 48 and 49 and by this means, for examplein the case of a drop in power-assisted brake pressure, can ensuresufficiently large flow quantities through the non-return valves 50 and51. It follows that the respective combinations of the 2/2-way valves 44and 45 and the throttles 48 and 49 can be used for varying the flowquantities through the non-return valves 50 and 51 as a function of theway in which the valve arrangements 37 to 40 are controlled. So as tomake this possible, the 2/2-way valves 44 and 45 have electromagnets 76which are connected, in a manner which is not shown, to the control unit36.

On the other hand, however, it is also possible to omit the throttles 48and 49, together with the by-passes 46 and 47, and to use the 2/2-wayvalves 44 and 45, which are closed in their basic position, to separatethe valve arrangements 37 to 40 hermetically from the power source 23while no power-assisted brake operation is taking place. To this extent,draining of pressure medium from the wheel brakes 8 to 11 in thedirection of the power source 23 is also avoided.

Because the power source 23 can be taken from the prior art, forexample, it is sufficient to indicate that the pressure monitor 28reports the degree of filling of the power reservoir 27 to the controlunit 36 so that the latter can fill the power reservoir 27 by switchingon the driving motor 24 and by driving the pump 25 and, on the otherhand, stops the increase in pressure in the power reservoir 27 andavoids overloading of the driving motor 24. After the driving motor 24has been switched off, the non-return valve 26 has the effect that thepower reservoir 27 cannot be emptied through the pump 25 in thedirection towards the reservoir 7. A safety valve 26a is installedbetween the power reservoir 27 and the reservoir 7 in order to secureagainst overloading of the driving motor 24.

Mode of operation of the power-assisted brake system

Let it be assumed, for the moment, that a supply of electric energy tothe control unit 36 is not taking place for some reason or other. Allthe directional valves represented in FIG. 1 are then located in thebasic positions shown. Hydraulic through connections therefore existbetween the emergency brake pressure source 6 and the wheel brakes 8 to11 through the emergency brake main conduits 12 and 13 and the emergencybrake conduits 14 and 15, 16, 17, the 2/2-way valves 54 and 70 locatedin these conduits being open, as already mentioned. On actuation of theemergency brake pressure source 6 by means of the pedal 3, it istherefore possible to make brake pressures available to the wheel brakes8 to 11. Depending on the weight of the vehicle equipped with such avehicle brake system and on the degree of comfort desired in emergencybraking operation, the vacuum-operated servo brake 5 may be associatedwith the emergency brake pressure source 6 or it may be omitted forprice reasons.

If the electrical energy supply is in order, the control unit 36 canconnect the driving motor 24 electrically to the electrical energysource quoted and, by this means, drive the pump 25 as soon as thevehicle equipped with the control unit 36 is made ready for operation bymeans of an ignition key. By means of pressure medium from the reservoir7, the pump 25 fills the power reservoir 27 and increases the pressurethere to a desired supply pressure. The attainment of the pressure ismonitored by means of the pressure monitor 28. The control unit 36 canbe activated to carry out power-assisted braking actions from a selectedlower pressure. This takes place when a driver actuates the pedal 3 sothat the required brake pressure value generator 35 reports adisplacement of the pedal rod 4 to the control unit 36. By this means,the control unit 36 recognizes that the wheel brakes 8 to 11 have now tobe isolated from the emergency brake pressure source 6 and have now tobe actuated by pressure medium from the power source 23 by means of thevalve arrangements 29 and 30. A displacement sensor is Used as therequired brake pressure value generator 35 in the present case. So thatsufficiently long pedal paths or displacements of the pedal rod 4 occurdespite the separation of the wheel brakes 8 to 11 from the emergencybrake pressure source 6, which can be designed in the manner of a mainbrake cylinder, the displacement simulator connecting valve 53 is openedby means of the electromagnet 66 so that the displacement simulator 52can take up quantities of pressure medium as a function of pressuresupplied, in a manner known per se. The driver can therefore generatepressures of greater or lesser magnitude in the displacement simulatoragainst the elastic resistance thereof; and the required brake pressurevalue generator 35, which is configured as a displacement sensor, actsin an indirect manner by means of the relationship between the pressurerespectively present in the displacement simulator 52 and theproportional displacement of the pedal rod 4. On the other hand,however, it is also possible to use a directly acting required brakepressure value generator 35a in the form of a pressure sensor inconnection with the emergency brake pressure generator 6, as in theprior art in accordance with the introduction to the description.

Apart from the wheel brakes 8 to 11 being shut off, as mentioned,relative to the emergency brake pressure source 6 when the pedal 3 isactuated, the wheel brakes 8 to 11 are also connected to the returnconduit 59 and, by this means, to the reservoir 7. In consequence, thewheel brakes 8 to 11 are initially unpressurized because of the normallyopen 2/2-way valves 70. This is indicated to the control unit 36 by thewheel brake pressure sensors 31, 32, 33 and 34. In consequence, thewheel brake pressure sensors 31 to 34 are used as actual brake pressurevalue generators. With the present assumption, the control unit 36recognizes that a brake pressure desired by the driver by actuation ofthe pedal 3 and requested by the required brake pressure value generator35 is lacking in the wheel brakes 8 to 11 which are in communicationwith the reservoir 7. In consequence, the control unit 36 drives the2/2-way valves 70 mentioned into their shut-off positions and opens theother 2/2-way valves 72 so that the wheel brakes 8 to 11 obtain pressuremedium from the power source 23 through the throttles 48 and 49 and the2/2-way valves 72, with the result that brake pressures occur and risein the wheel brakes 8 to 11. The respective magnitude of the brakepressure in each of the wheel brakes 8 to 11 is reported by associatedactual brake pressure value generators 31 and 32 and 33 and 34 to thecontrol unit 36. The latter then recognizes that originally presentdifferences between the required brake pressure values and actual brakepressure values are becoming smaller or have even disappeared. Once thedifferences have respectively become sufficiently small or havedisappeared, the control unit 36 terminates the supply of pressuremedium from the power source 23 to the respective wheel brakes 8 to 11by closing the respectively associated 2/2-way valve 72. This takesplace because the respective electromagnet 73 of the latter no longerreceives any control current. If the brake pedal 3 is held still, abraking effect is therefore set in the wheel brakes 8 to 11, thisbraking effect then remaining initially practically constant.

If the driver permits the pedal 3 to return at least partially in thedirection of the initial position, the magnitude of the signal reportedby the required brake pressure value generator 35 decreases and thecontrol unit 36 recognizes that the indications of actual brake pressurevalues from the actual brake pressure value generators 31 to 34 are nowlarger and that, therefore, brake pressures in the wheel brakes 8 to 11are instantaneously too high. The control unit 36 therefore drives the2/2-way valves 70, which have been held closed up to then by means ofthe electromagnets 71, back into their normally open position with theresult that quantities of pressure medium escape from the wheel brakes 8to 11 through the emergency brake conduits 14 to 7 and the emergencybrake main conduits 12 and 13 to the reservoir 7 so that the wheel brakepressures sink. The control unit 36 monitors the manner in which theactual brake pressure values now sink and how they approach theinstantaneously present required brake pressure value. When the approachis sufficiently close, the control unit 36 may then close the 2/2-wayvalves 70 again by means of the electromagnets 71. This is, for example,the case where there is a transition from full braking to partialbraking. If, however, the pedal 3 is released completely, the controlunit 36 recognizes this and it will no longer drive the 2/2-way valves70 into the closed positions when the pressures in the wheel brakes 8 to11 sink to the value "practically zero". In consequence, at least these2/2-way valves 70 are in the basic position which is used for emergencybrake operation, should this be necessary. Subsequently, the controlunit 36 also permits the 2/2-way valves 54, the 2/2-way valves 56 andthe displacement simulator connecting valve 53 to return to their basicpositions with the result that the two emergency brake main conduits 12and 13 between the emergency brake pressure sources 6 and the wheelbrakes 8 to 11 again provide passages for emergency brake operation andpressure medium drainage into the displacement simulator 52, which isundesirable in emergency brake operation, is prevented.

As is taught in the publication DE 31 31 856 A1, the vehicle hydraulicbrake system 2 of FIG. 1 can be further developed to carry out anti-lockprotection operation automatically. For this purpose, wheel rotationsensors 80 to 83 are allocated to the wheels (which are not represented)and these wheel rotation sensors 80 to 83 are connected to the controlunit 36, as is represented in the case of the wheel rotation sensor 83.The control unit 36 then additionally contains a logic circuit whichrecognizes, from sequences of wheel rotation signals from the wheelrotation sensors 80 to 83, whether a danger of the braked wheels lockingis present and which, in consequence, lowers brake pressures (separatelyand individually for each wheel brake 8 to 11, for example) in at leastone of the wheel brakes 8 to 11 to below such values as have beenpreselected by the driver by adjusting the required brake pressure valuegenerator 35. The 2/2-way valve 70 is again used for reducing the brakepressure. The difference from the previously described power-assistedbraking procedure now consists in the fact that when the danger of wheellock is present, the control unit 36 ignores the required brake pressurevalue which has been set too high by the driver and now controls thevalve arrangements 37 and 38 and 39 and 40 to achieve a variation inwheel rotation which avoids the danger of wheel lock. It is notnecessary to deal with the relevant details here because the design ofbrake systems for carrying out anti-lock protection operation is knownfrom many sorts of configuration examples.

While following the teaching of the prior art, this vehicle hydraulicbrake system can also be further developed to carry out drive-slipcontrol operation by selective braking of driving wheels, which are notrepresented. Because of the four arrangements of valve arrangements 37to 40 represented, this vehicle hydraulic brake system is also suitablewhere the driving wheels are associated with two different brakecircuits or where, for example, four driving wheels are provided on thevehicle.

Thanks to the four valve arrangements 37 to 40 distributed among fourwheels, further development of the control unit 36 makes this vehiclebrake system also suitable for carrying out vehicle dynamics control; itis possible to generate yaw direction changes influencing the drivingbehavior of the vehicle by braking individual wheels in accordance withthe prior art. On the other hand, however, it is also possible tospecify required brake pressure values which are different for the frontand rear wheel brakes on the basis of a stroke of the pedal 3 selectedby the driver and to set them by using the valve arrangements 37 to 40and the associated actual brake pressure value generators 31 to 34, asis also taught by the prior art. In addition, it is possible to designthe control unit 36 in such a way that vehicle decelerations occur whichare proportional to an adjustment of the pedal rod 4.

The principle, in accordance with the invention, of using at leastpartial lengths of emergency brake main conduits and the completelengths of the emergency brake conduits 14 and 15 and 16 and 17associated with the wheel brakes is not tied to the designs of the valvearrangements 29, 30 and 37 to 40 represented in FIG. 1. As an example,the valve arrangements 29 and 30 can be replaced by valve arrangements29a and 30a in the form of so-called 3/2-way valves 85 as shown in FIG.4. For this purpose, 3/2-way valves can be extracted from the prior art.In the basic position, the 3/2-way valve 85 connects the emergency brakepressure source 6 to the wheel brake 8 through, for example, theemergency brake main conduit 12.

In a manner which supports the invention, it is possible for a by-pass64 with a non-return valve 65, which can be opened to the emergencybrake pressure source 6, to be led around the 3/2-way valve 85, as hasalready been explained in the case of FIG. 1. The 3/2-way valve 85 can,for example, be controlled by means of an electromagnet 86. The choicebetween the valve arrangements of FIG. I and those of FIG. 4 is left tothe specialist.

The valve arrangement 37 represented in FIG. 1 can be replaced by avalve arrangement 37a, as shown in FIG. 5. In the latter case, aproportional directional valve 70a is provided instead of the 2/2-wayvalves already described with positions "normally open" and "drivenshut" and a proportional directional valve 72a is provided instead ofthe normally closed and driven open 2/2-way valve 72. The non-returnvalve 74 of the valve arrangement 37 can, for example, be appropriatelytaken over into the valve arrangement 37a and again laid as a by-passaround the proportional directional valves 70a. The proportionaldirectional valves 70a and 72a are valves whose positions can besteplessly varied between their end positions. The valve arrangement 37aof FIG. 5 can also be substituted for the valve arrangements 38, 39 and40. The advantages of joltless rates of brake pressure change, which canbe set to different levels as described in the publication DE 40 29 793A1, are possible when using the valve arrangement 37a with correspondingfurther development of the control unit 36.

A valve arrangement 37b, which is configured in the form of a so-called3/3-way valve with an electromagnet 87, can be used as an alternative tothe valve arrangements 37 and 37a represented in FIG. 1 and in FIG. 5.The 3/3-way valve 37b is therefore likewise suitable for increasingbrake pressure, maintaining brake pressure constant and reducing brakepressure. In order to achieve the advantage quoted for the valvearrangement 37a of FIG. 5, the valve arrangement 37b of FIG. 6 can beconverted into a 3/3-way proportional/directional valve arrangement 37cfor continuous transitions from one function into the respectivelyadjacent function of the three functions represented.

It may therefore be seen that the essential feature of theinvention--using emergency brake main conduits on a part of their lengthfor draining pressure medium from the power-assisted brakeoperation--can be realized with differently designed valve arrangements.As already indicated, this essential idea of the invention can also berealized independently of whether each of the wheel brakes 8 to 11 hasits own valve arrangement 37 or 38 or 39 or 40 associated with it orwhether, for example, only one common valve arrangement for changingwheel brake pressures is possible for the wheel brakes of two rearwheels, whether these changes to wheel brake pressures are used inpower-assisted brake operation or in anti-lock protection operation.

The foregoing relates to preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

We claim:
 1. A vehicle hydraulic brake system which comprises a twincircuit emergency brake pressure source which can be put into operationby a pedal, wheel brakes which can be supplied from the emergency brakepressure source, emergency brake main conduits laid as a twin circuitand extending from the emergency brake pressure source to the wheelbrakes and with a power-assisted brake device which can be controlledelectrically by means of the pedal, said brake device has at least onefluid pump which can be supplied from a reservoir, a power reservoirwhich can be charged by said at least one pump, and at least onecontrollable first valve arrangement (29, 30) per brake circuit, for theselection of a brake pressure source, between the emergency brakepressure source and wheel brakes and said brake device has at least oneelectrically controllable second valve arrangement (37, 38, 39, 40, 37a,37b, 37c), for brake pressure setting, a required brake pressure valuegenerator (35) which can be controlled by means of the pedal, and acontrol unit (36) connected to the required brake pressure valuegenerator (35), said control unit (36) controls the at least one secondvalve arrangement (37, 38, 39, 40, 37a, 37b, 37c in order to generatebrake pressures from the energy of the power source and to change thesebrake pressures, wherein the at least one first valve arrangement (29,30) for selecting the brake pressure sources (6, 23) is arranged to openthe associated emergency brake main conduits (12, 13) to a returnconduit (59) which leads to the reservoir (7), wherein the at least onesecond valve arrangement (37, 38, 39, 40, 37a, 37b, 37c) is connected tothe emergency brake main conduits (12, 13) and at least one of the wheelbrakes (8, 9, 10, 11) to set brake pressures while relaying a pressuremedium from the power source (23) and is arranged in such a way thatsaid at least one second valve arrangement connects the emergency brakemain conduits (12, 13) to the wheel brakes (8, 9, 10, 11) when noelectrical current is applied to the at least one electromagnet (71, 73,83) and separates the wheel brakes (8, 9, 10, 11) from the power source(23) and, in power-assisted brake operation, connects the wheel brakes(8, 9, 10, 11) to the power source (23) for the purpose of a brakepressure increase and to the emergency brake main conduits (12, 13),which can be opened to the reservoir (7), for the purpose of brakepressure reduction.
 2. The vehicle hydraulic brake system as claimed inclaim 1, wherein the at least one controllable first valve arrangement(29, 30) used for the selection of the brake pressure source has anormally open 2/2-way valve (54) in the emergency brake main conduits(12, 13) and has a normally closed 2/2-way valve (56) which is connectedto the emergency brake main conduit (12, 13) and can be opened to thereturn conduit (59).
 3. The vehicle hydraulic brake system as claimed inclaim 2, wherein the 2/2-way valves (54, 56) are configured as setvalves, wherein a connection (A) on a valve seat end of the normallyopen 2/2-way valve (54) is connected at the emergency brake pressuresource end to the emergency brake main conduits (12, 13), wherein aconnection on the valve seat end of the normally closed 2/2-way valve(56) is connected to the return conduit (59) and through the latter tothe reservoir (7) and wherein a by-pass (64) with a non-return valve(65) which can be opened in a direction towards the emergency brakepressure source (6) is arranged around the normally open 2/2-way valve(54).
 4. The vehicle hydraulic brake system as claimed in claim 1,wherein the at least one controllable first valve arrangement (29a) forthe selection of the brake pressure source is configured as a 3/2-wayvalve (85), wherein in a normal position of the latter, the emergencybrake pressure source (6) is connected to at least one of the wheelbrakes (8, 9, 10, 11) and, in the controlled position, at least one ofthe wheel brakes (8, 9, 10, 11) is connected to the reservoir (7)through a partial length of the emergency brake main conduits (12, 13)and the return conduit (59).
 5. The vehicle hydraulic brake system asclaimed in claim 4, wherein the 3/2-way valve (85) is configured as aseat valve and wherein a by-pass (64) which by-passes the 3/2-way valve(85) and which has a non-return valve (65) which can be opened towardsthe emergency brake pressure source (6) is arranged on the emergencybrake conduits (12, 13).
 6. The vehicle hydraulic brake system asclaimed in claim 1, wherein the at least one second valve arrangement(37, 38, 39, 40) for setting brake pressure in power-assisted brakeoperation comprises a normally open 2/2-way valve (70) between theemergency brake pressure source (6) and at least one of the wheel brakes(8, 9, 10, 11) and 6f a normally closed 2/2-way valve (72) between atleast one of the wheel brakes (8, 9, 10, 11) and the power source (23).7. The vehicle hydraulic brake system as claimed in claim 6 wherein the2/2-way valves are configured as proportional directional valves (70a,72a).
 8. The vehicle hydraulic brake system as claimed in claim 7,wherein the directional valve (70a, 72a, 37c) is configured as a seatvalve and wherein a by-pass (75) with a built-in non-return valve (74)is associated with this seat valve, for flow to take place through thenon-return valve (74) from the emergency brake pressure source (6) thenon-return valve opens towards at least one of the wheel brakes (8, 9,10, 11).
 9. The vehicle hydraulic brake system as claimed in claim 1,wherein the at least one second valve arrangement for setting wheelbrake pressure while using the power source (23) is configured as a3/3-way valve (37b) which connects the emergency brake pressure source(6) to at least one of the wheel brakes (8, 9, 10, 11) in the normalposition, which separates at least one of the wheel brake (8, 9, 10, 11)from the emergency brake main conduits (12, 13) and the power source(23) in a second position and connects the at least one of the wheelbrakes (8, 9, 10, 11) to the power source (23) in the third position.10. The vehicle hydraulic brake system as claimed in claim 9 wherein the3/3-way valve is configured as a 3/3-way proportional/direction valve(37c).
 11. The vehicle hydraulic brake system as claimed in claim 10,wherein the directional valve (70a, 72a, 37c) is configured as a seatvalve and wherein a by-pass (75) with a built-in non-return valve (74)is associated with this seat valve, for flow to take place through thenon-return valve (74) from the emergency brake pressure source (6) thenon-return valve opens towards at least one of the wheel brakes (8, 9,10, 11).
 12. The vehicle hydraulic brake system as claimed in claim 1,wherein the power source (23) is connected to the at least one secondvalve arrangement (37 and 38, 39 and 40, 37a, 37b, 37c) via a branch(41) and two subsequent non- return valves (50 and 51).
 13. The vehiclehydraulic brake system as claimed in claim 12, wherein one normallyclosed 2/2-way valve (44 or 45) is introduced between the branch (41)and each of the non-return valves (50 and 51).
 14. The vehicle hydraulicbrake system as claimed in claim 13, wherein by-passes (46 and 47) withbuilt-in throttles (48 and 49) are laid around the 2/2-way valves (44and 45).
 15. The vehicle hydraulic brake system as claimed in claim 1,wherein the emergency brake pressure source (6) can be connected to adisplacement simulator (52), which is effective in power-assisted brakeoperation, by a normally closed 2/2-way valve of a type of adisplacement simulator connecting valve (53).
 16. The vehicle hydraulicbrake system as claimed in claim 15, wherein the brake pressure valuegenerator (35) is configured as a pick-up, such as a displacementpick-up or pivoting angle pick-up, which is mechanically coupled to thepedal (3).
 17. The vehicle hydraulic brake system as claimed in claim 1,wherein the control unit (36) intended for power-assisted brakeoperation is further developed for an additional control of at least onefunctional mode of an anti-lock protection, drive-slip controloperation, braking force distribution between front and rear wheelbrakes, vehicle dynamics control and electrical deceleration controlproportional to an actuation of the pedal (30).